The U.S. National Security Agency is collecting online address books from Yahoo, Hotmail, Facebook, Gmail and other providers in order to map human relationships, The Washington Post reported Tuesday.

Former NSA contractor Edward Snowden supplied the newspaper with documents showing how the NSA gathers email contact lists and "buddy lists" from chat programs. The address book data is used to map the connection and latent connections between individuals.

On a typical day, the NSA collects about 500,000 buddy lists and inboxes (which seems to refer to address books), according to the documents. But the number is also sometimes higher. On one representative day mentioned in the documents, the NSA gathered 444,743 Yahoo address books, 105,068 Hotmail contact lists and 82,857 address books from Facebook, 33,697 from Gmail and 22,881 from other providers for a total of 689,246.

While address books usually contain email addresses and contact details, some of them can also contain physical address information, phone numbers and full names. The NSA collects buddy lists because they frequently contain data such as parts of messages, according to the document.

The information is collected in bulk at key Internet access points controlled by foreign telecommunications companies and allied intelligence services and the documents show that at least 18 collection points are used.

All of the data collection takes place outside of U.S. territory, but contact lists of U.S. citizens also cross the international collection points because their email could be sent via non-U.S. points. Email originating in the U.S. can also cross NSA collection points when citizens are abroad or traveling.

Two senior U.S. intelligence officials granted anonymity to speak to the Post declined to say how many address books of U.S. citizens have been collected by the NSA, but did not dispute that the number is likely to be in the millions or tens of millions, the paper reported.

The NSA collects the data abroad because neither Congress or the U.S. Foreign Intelligence Surveillance Court has authorized the collection. Such data collection would be illegal if carried out from the U.S., according to the newspaper's sources.

Encryption can protect against the siphoning of contact lists, but is not used by all providers. Yahoo, which seems to be the biggest NSA target, does not encrypt its webmail service by default, but it said it would turn on encryption by default in January following the revelations, according to the Post. But even when encryption is used in webmail, third-party clients may transmit information unencrypted, making the data vulnerable to snooping, it added.

The new revelations follow a series of disclosures from documents provided to news media by Snowden that have revealed, among other things, the NSA's efforts to defeat online encryption, its broad access toA Verizon customer data and a program in which it is collecting data on users of Internet servicesA provided by Microsoft, Google, Facebook, Yahoo and others.

News media that Snowden has provided documents to have said that there are more revelations to come.